AQA A Level Physics复习笔记5.2.3 Superconductivity

• All materials have some resistivity - even good electrical conductors such as copper and silver
• Resistance means that when electricity flows through a material, it heats up and the electrical energy is wasted as thermal energy
  • The resistivity of a material can be lowered by lowering its temperature

   

• If a material is cooled below a temperature called the critical temperature, its resistivity disappears entirely. It is now a superconductor
• Therefore, a superconductor (or superconducting material) is defined as

A material with no resistance below a critical temperature

• The critical temperature is defined as

The temperature at which a material becomes superconducting

• A common superconducting material is mercury
  • Mercury has a critical temperature of 4.2 K

   

• The electrical resistivity against temperature for a normal metal compared to a superconductor can be shown on the following graph:

Resistivity against Temperature graph for a superconductor vs. a normal metal

• Superconductivity is a property of only certain materials that have the characteristics above
• This temperature threshold is sometimes referred to as the transition temperature

• Superconductors are useful for applications that require large electric currents
• Therefore, they are useful for:
  • The production of strong magnetic fields
  • The reduction of energy loss / dissipation in the transmission of electric power

   

• Such applications which require these could be:
  • MRI scanners
  • Transformers & generators - for fewer fire risks
  • Motors
  • Monorail trains
  • Maglev (magnetic levitation) trains
  • Particle accelerators - need large magnetic fields to accelerate particles
  • Fusion reactors
  • Electromagnets
  • Power / electrical cables
  • Microchips

   

• Maglev trains require extremely strong electromagnets to levitate the train due to such a large mass
  • This means they can travel at extremely high speeds up to 603 km / h
  • Maglev train systems currently only exist in Japan, South Korea and China

   

Maglev trains use strong electromagnets attached to the train and rails to levitate